1. Field of the Invention
The present invention relates to an imaging optical system having a bending optical element.
2. Description of the Related Art
In recent years, mobile electronic devices which are designed mainly for taking still/moving photographic images, such as digital cameras (still-video cameras) and digital camcorders (motion-video cameras), and other mobile electronic devices which are designed to be capable of taking such photographic images as a subsidiary function, such as mobile phones equipped with a camera and tablet computers, etc., equipped with a camera, have become widespread, and there has been a demand to miniaturize the imaging units incorporated in these types of mobile electronic devices. In order to miniaturize an imaging unit, it is known to configure an optical system of an imaging unit as a bending optical system which reflects (bends) a light bundle using a reflection surface of a reflecting element (bending optical element) such as a prism or a mirror. An imaging optical system having at least one bending optical element (hereinafter also referred to as a “bending optical system”) is advantageous in achieving a reduction in thickness of the imaging unit, especially in the travelling direction of the incident light emanating from an object to be photographed.
On the other hand, in an imaging optical system, which can be reduced in thickness, the imaging optical system has also been required to have a small F-number. Designing the imaging optical system to have a small F-number usually causes the lens diameter to increase, thus being incompatible with the demand for slimming. In the case of an imaging optical system which includes a reflection surface (bending optical element) in a lens group that is provided closest to the object side, since a lens group that allows a light bundle having a large axial light-bundle effective diameter (axial light bundle diameter) to pass through lies on a post-bending optical axis (i.e., an optical axis optically behind the reflection surface), if the imaging optical system is designed to have a small F-number, the aforementioned lens group (lying on the post-bending optical axis) also increases in size, which inhibits a reduction in thickness of the imaging unit.
A non-circular lens element, which is formed as a circular lens element with an outer edge section thereof partly cut off to miniaturize the lens element (or reduce the diameter of the lens element), is known in the related art (disclosed in Japanese Unexamined Patent Publication Nos. 2006-267391, 2010-24376 and 2013-105049). However, the non-circular lens elements disclosed in the above-mentioned disclosures of the related art are each designed based on the technical idea of removing portions of the lens element through which only light rays that reach the outer side of the imaging surface pass due to the imaging surface (image pickup device/image sensor) of an imaging optical system, which includes such a non-circular lens, being rectangular (non-circular) in shape. Accordingly, in such non-circular lens elements of the related art, sections of the lens element through which only off-axis light rays reaching the outer side of the imaging surface pass and are positioned outside the axial light bundle effective radius are merely cut off. The axial light bundle effective radius is defined by the length of a perpendicular line which connects the optical axis with a point of intersection between an optical surface of the lens element and a light ray which passes through the outermost peripheral edge of the lens element among a group of light rays (light bundle) emerging from an object point on the optical axis and reaching an image point on the optical axis through the optical system. Hence, the reduction in diameter (miniaturization) of the lens (lens group) is not sufficient, and accordingly, the bending optical system cannot be sufficiently reduced in thickness even any such non-circular lens elements of the related art are adopted.